Thyroxine (3,5,3',5'-tetraiodothyronine, T4) promotes normal growth and development. It also regulates heat and energy production. T4 is released from the thyroid gland, the largest endocrine organ in the human body. The primary hormone released is T4 although T3 (3,5,3'-triiodothyronine) is also released in small quantities. Tyrosine residues in thyroglobulin (a glycoprotein scaffold containing many tyrosine residues) are iodinated to form mono- or diiodo-tyrosine which can then couple to form either T3 or T4.